The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
In many modern automotive transmissions, especially dual clutch transmissions (DCT's), a plurality of hydraulic actuators (operators) carry out commands that provide both a desired shift and shift sequence. In order to control such operators and confirm the attainment of a desired position, it is common practice to employ plural state or proportional linear position sensors that, in the former case, provide, for example, a signal that changes from a first state to a second state when a particular operator position has been achieved and, in the latter case, provide a signal that varies linearly (proportionally) between a first actuator position and a second actuator position.
Clearly the data from proportional sensors provides far more useful information as they not only indicate when multiple distinct actuator positions have been achieved but also provide the real time position of the actuator during translation and, if differentiated, the speed of the actuator. Because of these benefits, proportional sensors are by far the most commonly utilized linear position sensors and magnetic field sensors such as Hall effect sensors are the most common type.
In a typical magnetic field linear sensor assembly, a permanent magnet is mounted to a translating component such as the actuator piston, the output shaft or an associated shift rail and the magnetic sensor, which is stationary, is secured in proximate, sensing relationship with the permanent magnet to a housing, flange, web or other stationary transmission component. Translation of the permanent magnet thus varies the magnetic field strength sensed by the sensor and, with proper conditioning, scaling and software, the position of the actuator and associated shift components can be determined.
While accurate and dependable, the magnetic field strength sensor is not without drawbacks. Arguably the most problematic is its susceptibility to stray magnetic fields. A stray field can mimic the field produced by the associated permanent magnet such that the magnetic field sensor may provide a signal indicating that an actuator is in a certain position when it is not. Contrariwise, a stray magnetic field may interfere with the field from the permanent magnet and cause a sensor to indicate that an actuator and associated shift components have failed to achieve a desired position when, in fact, they have.
One approach to this problem is to provide shielding proximate the magnetic field sensor of materials such as mu metal which are designed to minimize stray magnetic fields and thus reduce inaccurate signals from the magnetic field sensor. Unfortunately, this solution adds weight and, in the case of an automatic motor vehicle transmission, occupies space in an already crowded environment. Furthermore, it is often difficult to sufficiently shield the sensor as certain regions must be left unshielded to allow the sensor to function properly.
From the foregoing, it is apparent that improvements in the art of magnetic field sensor isolation would be both desirable and beneficial. The present invention is so directed.